Here is a quick recipe for resizing an Online Multipath Device
Use the following procedure
Resize your physical device. This is storage platform specific.
Use the following command to find the paths to the LUN:
# multipath -l
Resize your paths. For SCSI devices, writing 1 to the rescan file for the device causes the SCSI driver to rescan, as in the following command:
# echo 1 > /sys/block/device_name/device/rescan
Resize your multipath device by running the multipathd resize command:
# multipathd -k 'resize map mpatha'
Resize the file system (assuming no LVM or DOS partitions are used):
# resize2fs /dev/mapper/mpatha